The overall goal of this grant has been to understand in molecular details the action of apolipoproteins (apos) and lipoproteins in health and disease. The current focus is on human apoA-I, the major protein of plasma high-density lipoproteins (HDL, a.k.a. good cholesterol) that remove cholesterol from cells and protect against cardiovascular disease. ApoA-I can be released from HDL in a labile lipid-poor/free state that is the precursor of amyloid, and can cause two forms of systemic human amyloidosis. In the acquired form, apoA-I deposits in the arteries as fibrils, which augments atherosclerosis. In the hereditary form, fragments of mutant apoA-I deposit in vital organs (kidney, liver, nerves, etc.) and damage them. There is no cure for this life-threatening disease and the only current treatment is organ transplant. To pinpoint therapeutic targets, we must understand what determines the dynamic equilibrium between the generation of the amyloid precursor, its clearance, and its misfolding, from the native helix-bundle structure in free apoA-I to the insoluble cross-?-sheet in amyloid. Unraveling the complex process of protein misfolding and proteostasis to block systemic amyloidosis has been very challenging but not impossible. To this end we have integrated high- and low-resolution structural and spectroscopic methods (circular dichroism, fluorescence, hydrogen-deuterium exchange, etc.) with biochemical and computational tools. Analysis of several disease-causing mutants enabled us to propose the first molecular mechanism of apoA-I misfolding. We postulated that perturbed packing in amyloid `hot spots' combined with the structural integrity of the native fold make the protein amyloidogenic. This idea can be extended to other pro- teins; it helps explain why structural destabilization of many globular proteins is neither necessary nor sufficient to cause amyloid disease. These and other new ideas will be rigorously tested in the next cycle of this grant. Aim 1 will elucidate the molecular mechanism of apoA-I misfolding in hereditary amyloidosis. We will determine how the disease-causing mutations perturb the native protein conformation in sensitive segments to promote ?-aggregation vs. proteolysis. Cell-based studies will unveil why mutation carriers are at a low risk of atherosclerosis despite low levels of plasma HDL. Aim 2 will identify the similarities and differences in the molecular basis for hereditary and acquired amyloidoses, and will establish the structure-toxicity relationship in aggregated forms of apoA-I. Aim 3 will define the role of lipids in apoA-I misfolding. This aim will test our hypothesis that lipid-lowering approaches hold therapeutic potential for apoA-I amyloidoses. This research opens a new frontier that goes beyond protein stability to identify key drivers of protein misfolding in vivo. The results will unveil the link between amyloidogenic and cardioprotective properties of apoA-I, help find therapies for apoA-I amyloidoses, yield sharper insights into the misfolding of other apos that are prominent in human amyloidoses, and have broad implications for misfolding diseases caused by other globular proteins.